A laminated glass including two glass plates integrated through an interlayer film for a laminated glass containing plasticized polyvinyl butyral is widely used, particularly, for vehicle windshields.
In a method for producing a laminated glass, for example, an interlayer film for a laminated glass unwound from a rolled body is cut into an appropriate size, and the resulting interlayer film for a laminated glass is sandwiched between at least two glass plates. The obtained laminate is placed in a rubber bag and vacuum suctioned so that the glass plates and the interlayer film are preliminarily pressure-bonded while air remaining therebetween is removed. Then, the laminate is pressurized with heat, for example, in an autoclave, to be finally pressure-bonded (e.g., Patent Literature 1).
In the production process of a laminated glass, deaeration properties upon lamination of a glass plate and an interlayer film for a laminated glass in preliminary pressure bonding are important. The interlayer film for a laminated glass therefore has a large number of protrusions and recesses on at least one surface thereof to ensure deaeration properties in the production of a laminated glass. Recently, an extrusion lip embossing method utilizing melt fracture has been employed for forming such protrusions and recesses (e.g., Patent Literature 2).
Protrusions and recesses formed on a surface of an interlayer film for a laminated glass are crushed upon pressure bonding of a glass plate and an interlayer film for a laminated glass after stacking thereof in the process from preliminary pressure bonding to final pressure bonding. They therefore are hardly visible in the obtained laminated glass. However, the protrusions and recesses sometimes cause optical distortion in the obtained laminated glass. In particular, in the case where the interlayer film for a laminated glass is formed of a laminate that includes two or more resin layers, occurrence of optical distortion is remarkable. The optical distortion of a laminated glass herein refers to a phenomenon where an object observed through a laminated glass looks distorted.
For reducing the optical distortion of a laminated glass, a method of controlling the pattern of protrusions and recesses on a surface of an interlayer film for a laminated glass has been employed (e.g., Patent Literature 3). In actuality, however, it is difficult to sufficiently prevent occurrence of optical distortion. Even when the optical distortion is reduced to the extent that the front view is visible without any difficulty, the optical distortion may remain to cause flickering of signals or pedestrian crossings.